They are not immune to outsiders...Not sure what your talking about? All Chickens carry disease of some type.
Huh? So what you're trying to say is if you get new hatched chicks from hatchery or feed store they are carrying Mereks or some other chicken disease. NO, they are not.
Or are you trying to say they can catch something from a person's existing flock? Why yes, yes they can. But....a person with an existing flock would know what they carry.